Up-conversion luminescence and near-infrared quantum cutting in Y 6O5F8:RE3+ (RE = Yb, Er, and Ho) with controllable morphologies by hydrothermal synthesis

Article abstract of DOI:10.1039/c2dt32463f

Monodisperse and uniform Y₆O₅F₈:RE ³⁺ (RE = Yb, Er, and Ho) microarchitectures with various morphologies have been constructed by a facile surfactant-assisted hydrothermal route, and their up-conversion luminescence and NIR quantum cutting properties were investigated. Hollow hexagonal prisms, microbundle gatherings by rods, and solid hexagonal prisms were designed by employing CTAB, PVP, and EDTA as additives, respectively. Under 980 nm excitation, the Y_5.34O₅F ₈:0.6Yb³⁺, 0.06Er³⁺ samples obtained using different additives exhibit similar emission spectra profiles with predominating peaks at 670 nm; the Y_5.34O₅F₈:0.6Yb ³⁺, 0.06Ho³⁺ samples give green emissions with the strongest peaks around 544 nm. The NIR quantum cutting for the Y ₆O₅F₈:Yb³⁺, Ho³⁺ samples was identified by the NIR emission spectra upon both 360 and 450 nm excitation. The corresponding quantum cutting mechanisms were discussed through the energy level diagrams, in which a back-energy-transfer from Yb³⁺ to Ho ³⁺ was first proposed to interpret the spectral characteristics. A modified calculation equation for the quantum efficiency of Yb ³⁺-Ho³⁺ coupled by exciting at 450 nm was suggested according to the quantum cutting mechanism. The efficient NIR luminescence and quantum cutting in Yb³⁺, Ho³⁺ co-doped Y₆O ₅F₈ reveal a possible application in modifying the solar spectrum to enhance the efficiency of silicon solar cells.

Full text of DOI:10.1039/c2dt32463f

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Graphical Abstract
Text: Various morphologies have been obtained hydrothermally by employing
different surfactants, which exhibit excellent upconversion and NIR quantum cutting
luminescence properties.

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Upconversion Luminescence and Near-infrared Quantum Cutting in
Y6O5F8:RE3+ (RE = Yb, Er, and Ho) with Controllable Morphologies by
Hydrothermal Synthesis
Jia Zhang, Yuhua Wang,* Linna Guo and Pongyu Dong
5
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
First published on the web Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
Monodisperse and uniform Y6O5F8:RE3+ (RE = Yb, Er, and Ho) microarchitectures with various
morphologies have been constructed by a facile surfactantꢀassisted hydrothermal route, and their
10 upconversion luminescence and NIR quantum cutting properties were investigated. Hollow
hexagonal prisms, microbundles gathering by rods, and solid hexagonal prisms were designed by
employing CTAB, PVP, and EDTA as additives, respectively. Under 980 nm excitation, the
Y5.34O5F8:0.6Yb3+, 0.06Er3+ samples obtained using different additives exhibit similar emission
spectra profiles with predominated peaks at 670 nm; the Y5.34O5F8:0.6Yb3+, 0.06Ho3+ samples give
15 green emissions with the strongest peaks around 544 nm. The NIR quantum cutting for
Y6O5F8:Yb3+, Ho3+ samples was identified by NIR emission spectra upon both 360 and 450 nm
excitation. The corresponding quantum cutting mechanisms were discussed through the energy
level diagrams, in which a backꢀenergyꢀtransfer from Yb3+ to Ho3+ was first proposed to interpret
the spectral characteristics. A modified calculation equation for quantum efficiency of Yb3+ꢀHo3+
20 couple by exciting at 450 nm was suggested according to the quantum cutting mechanism. The
efficient NIR luminescence and quantum cutting in Yb3+, Ho3+ codoped Y6O5F8 reveal the possible
application in modifying the solar spectrum to enhance the efficiency of silicon solar cells.
cross section in NIR region to make up for the lack of
matched energy levels with the 980 nm photon energy of the
activator ions. On the contrary, NIR QC involves downꢀ
conversion of one incident highꢀenergy photon to two or more
55 lowꢀenergy photons, which is extensively observed in RE ions
doped materials.9 To realize the NIR QC efficiently, other RE
ions are generally codoped with Yb3+ in the hosts, in which
the cooperative or resonant ET will occur. It is known that the
1. Introduction
The fabrication of nanoꢀ to microꢀscale inorganic structures
25 assembled by nanoparticles, nanorods/nanowires, and
nanobelts as building blocks with wellꢀdefined shapes and
inner structures has attracted great interest as a result of their
novel properties and application convenience.1, To date,
2
many efforts have been made to explore excellent synthetic
30 approaches to the fabrication of a variety of inorganic
functional materials with different levels to enhance their
performance in currently existing applications. The
hydrothermal method as a typical solutionꢀbased approach
was proven an effective and convenient process in preparing
35 various inorganic materials with diverse controllable
morphologies and architectures.3 Especially, the rare earth
(RE) ions doped fluorescent materials with monodisperse and
uniform morphologies have gained widely attention due to
their potential applications for many research issues, such as
40 upconversion (UC) and nearꢀinfrared (NIR) quantum cutting
(QC).
As is known, UC is a process where lowꢀenergy light, usually
NIR or infrared, is converted to higher energies, ultraviolet or
visible, via multiple absorptions or energy transfers (ETs).4, 5
45 Recently, it has been extensively investigated due to the
potential applications in the fields of solidꢀstate lasers,
multicolor three dimensional displays, optical storage, and
biological fluorescent labels.5ꢀ8 For the UC materials, the most
used activator ions are Ho3+, Er3+, and Tm3+, by coꢀdoping
50 Yb3+ which is an efficient sensitizer with large absorption
most studied RE ion couples are Tb3+ꢀYb3+, Tm3+ꢀYb3+, Ho3+
ꢀ
60 Yb3+, and Pr3+ꢀYb3+, and the matrixes are usually crystals,
glasses, or glass ceramics.9ꢀ14 For the time being, NIR QC has
become a hotꢀfocused research topic due to the promising
application in solar cells.15ꢀ17 As we know, the mismatch
between the solar spectrum and the band gap energy of silicon
65 semiconductor exists, and photons with energy lower than the
band gap cannot be absorbed, while for photons with energy
larger than the band gap, the excess energy is lost by
thermalization of hot charge carriers.18 Take these sources of
energy loss into account for the solar spectrum, the maximum
70 energy efficiency that can be reached is 30% for a crystalline
Si solar cell with a bandgap of 1.12 eV.9 Considering this, if
the conversion of one UV or visible photon into two NIR
photons with energies near to 1.12 eV is realized though QC
in a silicon solar cell, the energy loss related to thermalization
75 of hot charge carriers can be reduced and the solar cell
efficiency will be enhanced greatly.16
So far, high quantum efficiency (QE) for UC luminescence
and NIR QC are most obtained in fluorescent materials with
low phonon energy, such as fluoride phosphors. However,
80 fluorides are very sensitive to oxygen surface contamination,
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which may influence the luminescence properties and limit
2400 powder Xꢀray diffractometer (XRD) with Cu Kα
radiation (λ = 1.54178 Å) operating at 40 kV and 60 mA. The
morphologies of the asꢀprepared samples were inspected by
field emission scanning electron microscope (FESEM,
their application.19 Oxides generally show high chemical
stability and environmentalꢀfriendly characteristics, but
usually have higher phonon energy. Based on these, Y6O5F8
5
combining the advantages of both fluorides and oxides could 60 Hitachi, Sꢀ4800), transmission electron microscopy (TEM)
be an excellent host material for doping RE ions to realize the
effective application in solar cells. To the best of our
knowledge, the UC luminescence properties of Y6O5F8:Yb3+,
Er3+, Tm3+ were reported by Wang et al,20 but the
and highꢀresolution transmission electron microscopy (FEI
Tecnai F30, operated at 300 kV). The chemistry component
was analyzed by Fourier transform infrared spectroscopy (FTꢀ
IR, Nicolet NEXUS 670). The UC photoluminescence (PL)
10 morphologies of their samples are irregular, and the NIR QC 65 spectra were measured by using the 980 nm laser diode as
properties were not conducted in their work. In this paper,
RE3+ꢀdoped Y6O5F8 (RE = Yb, Er, and Ho) with various
excitation source, with the HORIBA Jobin Yvon Fluorlogꢀ3
Spectrofluorometer system. 0.3 double excitation
A
m
morphologies and multicolor were designed by
hydrothermal method, and their corresponding luminescence
15 properties of UC and NIR QC were investigated in detaied.
a
monochromator and two emission monochromators record the
emission spectra in the wavelength range of 200–850 nm
70 (with a HamamatsuR928 photomultiplier tube) or in the
wavelength range of 850–1650 nm (with a liquid nitrogenꢀ
cooled HamamatsuR5509ꢀ72 PMT). All the spectra
measurements were performed at room temperature.
2. Experimental
2.1 Preparation of Y(OH)CO3 and Y(OH)CO3:RE3+ (RE =
Yb, Er, and Ho) Precursors
3. Results and Discussion
In a typical synthesis of Y(OH)CO3, 0.7 mmol Y2O3 was
20 dissolved in dilute HNO3 solution (1:1 v/v) by heating to 80
°C with agitation to form clear Y(NO3)3 aqueous solution.
Superfluous HNO3 was evaporated by continuous heating
until the pH value of the Y(NO3)3 solution reaches about 4.
The 4.0 g of urea was dissolved in 40 ml of deionized water,
75 3.1 Structure and Morphology
3.1.1 Using CTAB as additive
The typical XRD pattern of Y(OH)CO3 precursor is presented
in Figure 1a. From the enlarged part, it shows two main broad
bands around 29° and 47°, implying its amorphous nature,
25 and then added to 40 ml of the above Y(NO3)3 solution drop 80 which is consistent with the XRD pattern for Gd(OH)CO3
by drop. The mixture was stirred continuously at room
temperature for half an hour, and then heated to 90 °C for 2 h
in the water bath. The resulting suspension was separated by
centrifugation and collected after washing with deionized
reported in Ref. 21. To further examine the chemical
composition of the precursor, FTꢀIR spectroscopy was carried
out, as shown in Figure 2a. The absorption bands centered at
3400 cmꢀ1, 1520 and 1400 cmꢀ1, 1080 cmꢀ1, 841 cmꢀ1, 750 and
30 water for three times. The RE3+ (RE = Yb, Er and Ho) doped 85 690 cmꢀ1 are attributed to OꢀH, νasOꢀCꢀO, CꢀO, πꢀCO32ꢀ, and
2ꢀ
23
Y(OH)CO3 were prepared by the same procedures except that
a stoichiometric amount of RE(NO3)3 was added to Y(NO3)3.
δꢀCO3
,
respectively.22,
This observation reveals the
presence of the hydroxyl and carbonate groups, which agrees
with the chemical formula of Y(OH)CO3 identified from the
XRD pattern.
2.2 Preparation of Y6O5F8 and Y6O5F8:RE3+ (RE = Yb, Er,
and Ho)
35 For a typical Y6O5F8 synthesis, the asꢀobtained Y(OH)CO3
precursor was dispersed into 30 ml of deionized water by
ultrasonic. Then 5.6 mmol of KFꢁH2O was dissolved in 30 ml
of deionized water which contains 0.1 g of certain dispersant
(i.e. Hexadecyl trimethyl ammonium Bromide (CTAB) or
40 Polyvinylpyrrolidone (PVP) or Ethylenediaminetetraacetic
acid (EDTA)), and dripped into the above suspension
followed by further stirring. After the mixture was agitated at
room temperature for half an hour, it was transferred into a
100 ml Teflonꢀlined, stainless steel autoclave containing 80
45 ml of suspension and heated at 210 °C for 48 h. After cooling
down naturally to room temperature, the product was
separated from the reaction media by centrifugation, washed
several times with deionized water and ethanol, and finally
dried overnight at 50 °C. The Y6(1ꢀx)O5F8:xRE3+ (RE = Yb, Er
50 and Ho) were prepared by the same procedure except for
using Y(OH)CO3:RE3+ as precursors. The heatꢀtreatment for
the hydrothermal product was carried out at 650 °C for 2 h
with a heating rate of 75 °C hꢀ1.
90
Figure 1 XRD patterns of (a) Y(OH)CO3, (b) Y (OH)1.63F1.37, and (c)
Y6O5F8.
The hydrothermal product by using CTAB as additive is white
powder, and crystallized into Y(OH)1.63F1.37 phase perfectly
2.3 Characterization
95 (agreeing with the JCPD Standard Card No. 80ꢀ2007, Figure
55 The phase purity was determined by using a Rigaku D/MAXꢀ
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1b). This hydrothermal product could be transformed into
pure Y6O5F8 phase after annealing at 650 °C for 2 hours,
reflected from the XRD pattern in Figure 1c which is well
agreement with the JCPD Standard Card No. 80ꢀ1125. The
treatment at 650 °C, the monodisperse and uniform
morphology features were maintained (Figure 3d) in spite of
the phase transition from Y(OH)1.63F1.37 to Y6O5F8. The size
of the hollow hexagonal prisms is almost the same as that
5
broad diffraction peaks indicate nanocrystallines may be 35 before annealing, but the surface has become very rough
generated in the hydrothermal process, and by DebyeꢀScherrer
formula,22 the average crystalline size of Y6O5F8 is estimated
to be about 55 nm. The FTꢀIR spectroscopy of Y(OH)1.63F1.37
in Figure 2b exhibits the typical OꢀH stretching (3650 and
(Figure 3e and 3f). Under higher magnification in the inset of
Figure 3f, small submicron particles between 100 and 200 nm
are found to consist of the microtubes.
10 3400 cmꢀ1) and YꢀO bond (513 cmꢀ1), but the OꢀH stretching
has almost disappeared after annealing due to the phase
transition (Figure 2c), remaining only the strong absorption
peaks ascribed to YꢀO bond (532 and 450 cmꢀ1),23, 24 and this
result is in agreement with the phase analysis of XRD in
15 Figure 1.
40
Figure 4 TEM images of (a) Y(OH)CO3 and (b, c) Y6O5F8; EDX
spectrum of Y5.34O5F8:10%Yb3+, 1%Er3+, inset shows the HRTEM image.
To further investigate the morphological characteristics of
Y6O5F8, TEM measurement was carried out. Figure 4a shows
the TEM image of the Y(OH)CO3 precursor, which exhibits
Figure 2 FTꢀIR spectra of (a) Y(OH)CO3, (b) Y (OH)1.63F1.37, and (c)
Y6O5F8.
45 spheroidal morphology. When the reactant was treated
hydrothermally for 15 h and annealed at 650 °C for 2 h
afterwards, the hollow structure has been achieved on the
whole, but some initial nanoparticles originating from
Y(OH)CO3 precursor still adhere to the surface of the
50 microtubes (Figure 4b). However, when the hydrothermal
time prolongs to 48 h, a final product of Y6O5F8 with smooth
surface has been obtained, as presented in Figure 4c. From the
enlarged edge of the tube in the inset of Figure 4c, submicron
particles ranging from 100 to 200 nm are found to consist of
55 the microtubes, which is agreement with the observation in
the SEM images. Based on this, we can predict that the initial
spheroidal particles of the Y(OH)CO3 precursor were
assembled to the final microtubes, and this conclusion can be
further verified by the timeꢀdependent experiments below.
60 The energyꢀdispersive Xꢀray (EDX) analysis spectrum of
Y5.34O5F8:10%Yb3+, 1%Er3+ given in Figure 4d shows that all
the composing elements of the sample have been detected, and
no other impure elements are found. The HRTEM image at
the edge of the microtubes is presented in the inset of Figure
65 4d. The lattice fringes are clearly visible with a spacing of
0.317 nm, agreeing well with the lattice spacing of (161) of
orthorhombic Y6O5F8.
20 Figure 3 SEM images of both (a, b, c) Y (OH)1.63F1.37 and (d, e, f) Y6O5F8.
Figure 3 presents the SEM images of both Y(OH)1.63F1.37 and
Y6O5F8. Figure 3a shows that uniform microrods were
obtained in the hydrothermal process. And from the enlarged
images of Figure 3b and 3c, it can be observed clearly that the
25 rods exhibit regular hexagonal prism shape with hollow
structure. The average length and outer diameter of the hollow
hexagonal prisms are estimated to be about 12 and 1.3 ꢂm,
respectively, and the wallꢀthickness is about 250 nm.
Moreover, every hollow hexagonal prism takes on very
30 smooth outer surface but irregular inter surface. After heatꢀ
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for 2 h. At the initial stage within 3 h, the product exhibits
agglomerated submicron particles, and almost no rods are
formed. When the reactant is given hydrothermal treatment
25 for 8 h, a lot of hollow hexagonal prisms start to be produced
by assembling the initial nanoparticles. After 15
h
hydrothermal reaction, most of the hollow hexagonal prisms
have been obtained except for some remaining nanoparticles
adhering to the surface of the prisms. After 24 h, the residual
30 nanoparticles were reduced further. For 48 h, the reaction has
been completed entirely, and monodisperse hollow hexagonal
prisms were formed finally. In the preparation of hollow
structure, CTAB plays an important role. The solution of the
CTAB contains hydrophobic CꢀN organic group,25 which
35 could chelate metal ions and hinder particle aggregation. The
dispersed particles accumulate and form the microtubes
though a reconstruction process. Similar hollow structure and
interpretation can be found in Ref. 26. The Y(OH)1.63F1.37
sample before annealing demonstrates a similar morphology
40 evolution process (Figure S3), indicating that heatꢀtreatment
has no visible effect on the sizes and shapes for samples
transfering from Y(OH)1.63F1.37 to Y6O5F8 phases.
3.1.2 Using PVP as additive
To study the effect of different additives on the morphology,
45 the samples with PVP as dispersant were prepared. The XRD
investigation indicates that the obtained sample of either
Y(OH)1.63F1.37 by hydrothermal route for 48 h (Figure S4d) or
Y6O5F8 annealed at 650 °C for 2 h afterwards (Figure S5d) is
singleꢀphase. The typical SEM images of both Y(OH)1.63F1.37
50 and Y6O5F8 are given in Figure 6(a, b) and (c, d), respectively.
Good dispersion and uniform shapes are obtained for both the
samples. From enlarged images of Figure 6(b, d), numerous
rods are observed to assemble to monodisperse bundles with
the average length and diameter of about 4 and 1.5 ꢂm,
55 respectively. The above result reveals that heatꢀtreatment does
not cause any change in size and shape of the samples, but the
surface of every rod has become rough after annealing.
Figure 5 SEM images of evolution for Y6O5F8 at different hydrothermal
times: (a, b) 3 h, (c, d) 8 h, (e, f) 15 h, (g, h) 24 h, and (i, j) 48 h.
To disclose the growth process of the hollow hexagonal
prisms, timeꢀdependent experiments were conducted by
keeping other reaction parameters unchanged. The
hydrothermal times are selected to be 3, 6 , 15, 24, and 48 h,
and the corresponding XRD patterns are given in Figure S1
for Y (OH)1.63F1.37 and Figure S2 for Y6O5F8. When the
5
10 reaction time is for 3 h, the hydrothermal product exhibits
very poor crystallinity (Figure S1a), and it crystallized into
YOF phase (not the desired product of Y6O5F8) after
annealing at 650 °C for 2 h (agreeing with the JCPD Standard
Card No. 71ꢀ2100, Figure S2a). When the hydrothermal time
15 exceeds 8 h, pure phase of either Y(OH)1.63F1.37 or Y6O5F8
with perfect crystallinity has been obtained (Figure S1bꢀS1e
and Figure S2bꢀS1e), implying the desired products were
being forming gradually. Figure 5aꢀ5j present the SEM
images of the intermediates of Y6O5F8 with various
20 hydrothermal times ((a, b) for 3 h, (c, d) for 8 h, (e, f) for 15
h, (g, h) for 24 h, and (i, j) for 48 h) after annealing at 650 °C
Figure 6 SEM images of both (a, b) Y(OH)1.63F1.37 and (c, d) Y6O5F8.
60 To reveal the corresponding morphological evolution, a series
of timeꢀdependent experiments were carried out and the
intermediate products collected with different reaction
intervals after annealing at 650 °C for 2 h were then
investigated by SEM (Figure 7). At the first stage of the
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hydrothermal process within 5 h, separated microrods are
hydrothermal reaction for 48 h and Y6O5F8 after annealing at
produced gradually from numerous nanoparticles of the 30 650 °C for 2 h afterwards, which reveals the asꢀprepared
Y(OH)CO3 precursor as can be seen in Figure 7(a, b). When
the hydrothermal time is prolonged to 10 h, the separated
microrods start to aggregate into the embryo bundles (Figure
7(c, d)). After 21 h, the monodisperse bundles have been
samples are singleꢀphase. The SEM images of Y(OH)1.63F1.37
are given in Figure 8(a, b). Uniform and monodisperse prismꢀ
like morphology was formed in the hydrothermal reaction,
and these prisms with the average length of 12 ꢂm and
5
formed on the whole except for some remaining nonꢀ 35 diameter of 3.5 ꢂm are observed to consist of several thin
assembled rods (Figure 7(e, f)). The reaction is entirely
completed after 48 h, as displayed in Figure 7(g, h). On the
10 whole, the samples tend to form long rods or tubes in the
initial reaction process in the hydrothermal environment as
microrods. Figure 8(cꢀf) show the SEM images of Y6O5F8. It
can be seen that the dispersion, shape, and size hardly change
except that the surface of every prism gets very rough after
annealing, which is composed of numerous submicron
witnessed in the SEM images regardless of surfactants, which 40 particles (Figure 8f). In addition, distinguishing hexagonal
is in agreement with the result in Ref. 20. But in the later
reaction stage with PVP present, the formed rods aggregated
15 to many bundles, and we predit this process could result from
the selfꢀaggregation or selfꢀassembly of the initial rods maybe
prism profile can be observed in Figure 8e, and every
hexagonal prism is formed by gathering several microrods
(Figure 8d), similar to that of Y(OH)1.63F1.37 in Figure 8b. The
timeꢀdependent experiments of the solid hexagonal prisms
due to the noncovalent interactions of long chain molecules of 45 indicate the formation process is similar to those in Figure 5
PVP attached to the surfaces of microcrystals. The
Y(OH)1.63F1.37 product without annealing shows similar
20 morphology evolution process in the hydrothermal reaction
(Figure S6), indicating the morphologies were not destroyed
in the heatꢀtreatment.
and 7, and a schematic illustration of Y(OH)1.63F1.37 or Y6O5F8
by using EDTA as additive is depicted in Scheme 1 to
interpret the formation mechanism. Microrods are generated
in the initial stage from the Y(OH)CO3 precursor (a and b).
50 Then the existing microrods start to gather gradually into the
basic hexagonal prism structure (c). At last, the uniform solid
hexagonal prisms are formed completely with the prolonged
reaction time (d). This selfꢀaggregation or selfꢀassembly
process could be owing to the excellent chelating and capping
55 ability of EDTA, since EDTA possesses four carboxyl groups
(ꢀCOOH) and two lone pairs of electrons on two nitrogen
atoms which can act as binding sites and help to form
hydrogen bonds, facilitating the selfꢀaggregation or selfꢀ
assembly process.27
Figure 7 SEM images of evolution for Y6O5F8 at different hydrothermal
60
25
times: (a, b) 5 h, (c, d) 10 h, (e, f) 21 h, and (g, h) 48 h.
Figure 8 SEM images of both (a, b) Y(OH)1.63F1.37 and (c, d, e, f) Y6O5F8.
3.1.3 Using EDTA as additive
In this system, EDTA is also used to control the morphology.
Figure S7 shows the XRD patterns of both Y(OH)1.63F1.37 with
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twoꢀphoton process, which agrees with the twoꢀstep sequential
4
4
transitions from the I15/2 ground state to I11/2 intermediate
state, then to the F7/2 state of Er3+ in the excitation process.29
4
40 On the other hand, a typical saturation phenomenon for
relative high power pump (≥ 370 mW) appears in the inset (a)
of Figure 9. The Y5.34O5F8:10%Yb3+, 1%Er3+ samples
prepared by using PVP and EDTA as additives demonstrate
similar spectra profiles (Figure S8 and Figure S9,
45 respectively) with the chromaticity coordinates of (0.678,
0.319) and (0.663, 0.334) for 370 mW, respectively, which
displays that the different morphologies hardy effect the UC
luminescence properties.
Scheme 1 Schematic illustration of the growth process of Y(OH)1.63F1.37
or Y6O5F8 by using EDTA as additive.
3.2 UC Luminescence of Y6O5F8:Ln3+ (Ln = Yb, Er, and Ho)
Figure 10 shows the UC emission spectra of
50 Y5.34O5F8:10%Yb3+, 1%Ho3+ using CTAB as additive under
DL excitation of 980 nm at various pump powers. The
predominated emission peaks are located at about 544 nm,
ascribed to the (5F4, 5S2)ꢀ5I8 transition of Ho3+. The two
relatively weak emission peaks around 651 and 753 nm could
55 be assigned to 5F5ꢀ5I8 and (5F4, 5S2)ꢀ5I8 transitions of Ho3+
,
respectively. Similarly, all the emission peaks exhibit
enhanced intensities with increasing excitation power. By
studying the pump power dependence of the UC emissions
according to eqn (1), the n values for the three emission peaks
60 of 544, 651, and 753 nm are calculated to be about 1.55, 1.77,
and 1.49, respectively (inset (a) of Figure 10). Hence, the
main UC mechanism for Yb3+ꢀHo3+ pairs in this host is a twoꢀ
photon process, that is, the twoꢀstep sequential transitions
5
Figure 9 UC emission spectra of Y5.34O5F8:10%Yb3+, 1%Er3+ using CTAB
as additive under DL excitation of 980 nm, inset (a) shows dependence of
UC emission intensity on excitation powder, inset (b) shows UC
fluorescence photograph of Y5.34O5F8:10%Yb3+, 1%Er3+ under a 980 nm
DL excitation.
5
5
from the I8 ground state to I6 intermediate state, then to the
65 5F4 state of Ho3+
The chromaticity coordinates were
.
10
calculated to be (0.327, 0.662) for 370 mW, indicating a
yellowish green color. The corresponding UC fluorescence
photograph of Y5.34O5F8:10%Yb3+, 1%Ho3+ dispersed into
ethanol solvent under 980 nm DL excitation is presented in
70 the inset (b) of Figure 10. The Y5.34O5F8:10%Yb3+, 1%Ho3+
samples prepared by using PVP and EDTA as additives
demonstrate similar spectra profiles, implying the different
morphologies hardy effect the UC luminescence properties.
Figure
Y
9
displays the UC emission spectra of
5.34O5F8:10%Yb3+, 1%Er3+ by using CTAB as additive under
DL excitation of 980 nm at various pump powers. Three
emission peaks around 525, 548, and 670 nm are observed,
15 attributed to H11/2ꢀ4I15/2, 4S3/2ꢀ4I15/2, and F9/2ꢀ4I15/2 transitions
of Er3+, respectively. The longꢀwavelength emission at 670
nm is predominated, much more intense than other emissions.
The corresponding chromaticity coordinates calculated from
the emission spectrum for 370 mW by the method using the
20 1931 CIE (Commission International de l’Eclairage France)
system are (0.642, 0.354), indicating a good red color purity.
The inset (b) of Figure 9 shows UC fluorescence photograph
of YOF:10%Yb3+, 1%Er3+ dispersed into ethanol solvent
under 980 nm DL excitation, which shows an intense red
25 color. Besides, from the spectra, it is obvious that the
emission intensities become stronger with enhanced pump
powers. It is well known that for unsaturated UC processes,
the relationship between the UC emission intensity I and the
pump power P could be expressed as follows:28
2
4
30 I f ∝ Pn (1)
where If is the fluorescent intensity, P is the pump power, and
n is the number of photons required to populate the emitting
state. For the strongest emission peak at 670 nm, a plot of lnIf
versus P yields a straight line with slope n for relative low
35 power pumps (< 370 mW), as shown in the inset of Figure 9,
and the n value obtained equals to about 1.49. Hence, it is a
75
Figure 10 UC emission spectra of Y5.34O5F8:10%Yb3+, 1%Ho3+ using
CTAB as additive under DL excitation of 980 nm, inset (a) shows
dependence of UC emission intensity on excitation powder, inset (b)
shows UC fluorescence photograph of Y5.34O5F8:10%Yb3+, 1%Ho3+ under
980 nm DL excitation.
6
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3.3 NIR QC of Y6O5F8:Yb3+, Ho3+
Figure 12 presents the emission spectra of Y6(1ꢀx)O5F8:xYb3+,
1%Ho3+ (0 ≤ x ≤ 15%) upon 450 nm excitation. In visible
The possible NIR QC mechanisms for Ho3+ꢀYb3+ couple are
5
region, with increasing Yb3+ content, the intensities of F3ꢀ5I8
31
shown in Figure 11.30,
Firstly, for excitation at 450 nm,
5
5
(488 nm), (5F4, S2)ꢀ5I8 (544 nm), F5ꢀ5I8 (651 nm), and (5F4,
30 5S2)ꢀ5I7 (755 nm) transitions of Ho3+ decrease heavily,
implying an efficient ET from Ho3+ to Yb3+. The excitation
three feasible ET processes from Ho3+ to Yb3+ could occur,
giving rise to NIR emission of Yb3+: (1) cooperative ET
(CET) process (CET in Figure 11): Ho3+(5F3)→2Yb3+(2F5/2) +
hν; (2) resonant crossꢀrelaxation process from 5S2, 5F4 states
populated from 5F1 state nonradiatively (CR1 in Figure 11):
5
spectrum by monitoring at 980 nm of Yb3+ F5/2ꢀ2F7/2
2
transition (inset of Figure 12) also confirms this efficient ET,
since it shows all the excitation peaks of Ho3+. In NIR region,
35 two strong emission peaks are observed around 980 and 1190
5
Ho3+(5S2, F4) + Yb3+(2F7/2)→Ho3+(5I6) + Yb3+(2F5/2) + hν; (3)
10 resonant crossꢀrelaxation process from 5F5 state populated
nonradiatively from higher energy levels (CR2 in Figure 11):
2
nm, attributed to Yb3+ F5/2ꢀ2F7/2 transition and Ho3+ 5I6ꢀ5I8
transition, respectively,30 and the former intensity is much
Ho3+(5F5)
+ + ) + hν.
Yb3+(2F7/2)→Ho3+(5I7) Yb3+(2F5/2
weaker than the latter one. Furthermore, the optimal Yb3+ꢀ
Secondly, for excitation at 360 nm, one ultraviolet photon
absorbed by Ho3+ could be converted into two Yb3+ NIR
15 photons through twoꢀstep sequential crossꢀrelaxations (CR3
2
doping concentration is found to be 5 mol% for Yb3+ F5/2
ꢀ
2
40 F7/2 transition, smaller than 10 mol% for Ho3+ 5I6ꢀ5I8
transition. These results seem contradictory to the mentioned
NIR QC mechanisms, with the detailed discussion shown
below. In Figure 11, the emission around 980 nm results from
both the CET and CR1 processes, and the emission of Ho3+
45 5I6ꢀ5I8 is mainly followed by the occurrence of CR1 process.
And, the CET process is known to be inefficient at lower Yb3+
content due to its intrinsic nature, but at a relatively high Yb3+
concentration of 10 mol%, the CET process could become
efficient, and this efficiency (ηCET) can be estimated by the
50 following equation:10
5
and CR4 in Figure 11) due to that the energy gap between G4
and 5F5, 5F5 and 5I7 are resonant with the energy of the
2
2
transition from F7/2 to F5/2 on Yb3+. In this work, a series of
Y6(1ꢀx)O5F8:xYb3+, 1%Ho3+ (0 ≤ x ≤ 15%) samples were
20 prepared to study the NIR QC properties of Yb3+ꢀHo3+ couple
in Y6O5F8 host, which could have potential to promote the
energy efficiency of crystalline silicon cells.
∫ Ix%Ybdt
∫ I0%Ybdt
ηET
=
ηx%Yb =1−
(2)
where I denotes the decay intensity, and x%Yb denotes the
Yb3+ contents. To determine the ηCET for Y5.34O5F8:10%Yb3+,
1%Ho3+ as a typical sample, decay curves with 450 nm
55 excitation and 487 nm emission for Y6(1ꢀx)O5F8:xYb3+,
1%Ho3+ (x = 0% and 10%) were shown in Figure 13a. Both
the decay curves demonstrate doubleꢀexponential feature, and
the average lifetimes are determined by the following
equation:
Figure 11 Energy levels diagram of Ho3+ꢀYb3+ couple showing possible
60
<
τ
> = (Aτ12 + A2
τ
22 ) / (Aτ1 + A2τ2
)
(3)
1
1
25
mechanisms for NIR QC processes.
Figure 12 Emission spectra of Y6(1ꢀx)O5F8: xYb3+, 1%Ho3+ (0 ≤ x ≤ 15%), inset shows excitation spectrum of Y5.34O5F8:10%Yb3+, 1%Ho3+ by monitoring at
980 nm.
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The corresponding lifetime values are summarized in Table 1.
35 ηQE =1+ηCET + (1−ηCET )ηCR1 (4)
Thus, ηCET was calculated to be 72.2%, which indicates that
5
the CET process as an extra decay pathway for energies at F3
where the last two terms stand for the extra QEs for CTE and
CR1 processes, respectively. The total ηQE for
Y5.34O5F8:10%Yb3+, 1%Ho3+ is calculated to 194.6% by this
formulation. The so high QE partly results from the Ho3+ 5I6ꢀ
40 5I8 emission (1190 nm), which is useless to enhance the
efficiency of Si solar cells, since this emission does not match
the spectral response of Si solar cells.
level is very efficient. In this case, the number of the
5
5
remaining photons relaxed to the lower energy levels than F3
to give other emission is small. Besides, the resonant ET
efficiencies for CR1 (ηCR1) and CR2 (ηCR2) calculated by eqn
(2) are 80.7% and 65.2%, respectively, which are also
efficient (the corresponding decay curves and lifetimes are
10 given in Figure 13(b, c) and Table 1). All the above results
For another NIR QC mechanism of CR3 and CR4 in Figure
11, the corresponding spectral characteristics of Y6(1ꢀ
45 x)O5F8:xYb3+, 1%Ho3+ (0 ≤ x ≤ 15%) were also investigated.
Figure 14 presents their excitation spectra by monitoring at
544 nm of Ho3+ emission, which have been normalized to the
excitation peak at 450 nm. All the excitation peaks could be
ascribed to the characteristic 4fꢀ4f transitions of Ho3+. It can
50 be seen that the intensities of the excitation peaks
2
indicate that the Yb3+ F5/2ꢀ2F7/2 emission should demonstrate
stronger intensity and higher quenching concentration than
those of Ho3+ 5I6ꢀ5I8 emission. However, the NIR emission
spectra in Figure 12 show distinct results. Based on these, we
15 predict that a back ET process from Yb3+ to Ho3+ has occurred
by Yb3+(2F5/2) + Ho3+(5I8)→Yb3+(2F7/2) + Ho3+(5I6) + hν, since
it was reported that the back ET phenomenon widely exists in
RE ions multiꢀdoped fluorescent materials.32ꢀ34 This ET not
only increases the Ho3+ 5I6ꢀ5I8 emission intensity but also
5
corresponding to the transition to the G4 level as well as the
upper levels (from 300 to 400 nm, denoted as Part A) decrease
2
20 reduces the Yb3+ F5/2ꢀ2F7/2 emission intensity, leading to the
5
gradually compared with those below the G4 levels (from 400
spectral features in Figure 12.
to 500 nm, denoted as Park B) with increasing Yb3+
55 concentration, whereas the excitation peaks inside Parts A and
B undergo almost no change in their relative intensities. This
Table 1 Decay lifetimes of YOF: xYb3+, 1%Ho3+ (0 ≤ x ≤ 15%).
sample
x = 0%
λex/λem (nm)
450/487
450/544
450/651
360/487
360/544
450/487
450/544
450/651
360/487
360/544
τ1 (ꢂs)
53.0
55.8
34.6
52.1
47.2
9.7
τ2 (ꢂs)
146.6
140.4
124.1
143.5
133.0
57.2
37.5
56.4
58.3
53.8
τ (ꢂs)
119.1
112.8
96.9
116.2
110.7
33.0
21.8
33.7
36.7
34.3
5
intensity decrease, when excitation is not lower than the G4
level, is due to the presence of CR3 in Figure 11, which is an
evidence for the occurrence of NIR QC. The efficiency of this
60 crossꢀrelaxation process can be expressed as31
x = 10%
(A/ B)x%Yb
9.8
ηCR =1−
(5)
10.1
15.2
10.0
(A/ B)0%Yb
where A/B is the ratio of the integrated excitation intensity
between Parts A and B, the subscript 0% and x% refer to the
Ho3+ single doped sample and the Ho3+ꢀYb3+ codoped
65 samples, respectively. According to eqn (5), the ηCR values for
x = 5%, 10%, and 15% are calculated to 12.3%, 21.0%, and
33.1%, respectively.
25
Figure 13 Decay curves of Y5.34O5F8:10%Yb3+, 1%Ho3+.
The QE is known to be defined as the radio of the number of
photons emitted to the number of photons that are absorbed. It
can be understood from Figure 11, that the extra photons
emitted come only by CET and CR1 process for every phonon
30 absorbed when excited at 450 nm. Therefore, supposing that
all the excited Yb3+ and the residual excited Ho3+ decay
radiatively, a modified calculation equation for the total QE
(ηQE) when excited at 450 nm can be theoretically expressed
as follows according to Ref. 10:
Figure 14 Excitation spectra of Y6(1ꢀx)O5F8:xYb3+, 1%Ho3+ (0 ≤ x ≤ 15%)
70
by monitoring at 544 nm.
The emission spectra of Y6(1ꢀx)O5F8:xYb3+, 1%Ho3+ (0 ≤ x ≤
8
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15%) in visible region upon 360 nm excitation are presented
in Figure 15, which reveal similar regularity in intensities and
spectral profiles to those in Figure 12, indicating an efficient
Ho3+ codoped Y6O5F8 samples, NIR QC was identified upon
360 and 450 nm excitation. A back ET from Yb3+ to Ho3+ was
first proposed to interpret the emission spectral
ET from Ho3+ to Yb3+. The emission spectra of Y6(1ꢀ 45 characteristics. The novel Y6O5F8:Yb3+, Ho3+ phosphors could
5
x)O5F8:xYb3+, 1%Ho3+ (5% ≤ x ≤ 15%) in NIR region were
given in the inset of Figure 15. It shows two strong emission
explore a new approach to maximize the performance of
silicon solar cells.
2
peaks at 980 and 1190 nm, attributed to Yb3+ F5/2ꢀ2F7/2 and
Ho3+ 5I6ꢀ5I8 transitions, respectively. And the quenching
concentration was observed to be 10%. According to the NIR
Acknowledgments
This work was supported by the National Science Foundation
50 for Distinguished Young Scholars (No. 50925206). We
express thanks to professor Xueyuan Chen and doctor
Haomiao Zhu of the Fujian Institute of Research on the
Structure of Matter for their assistance in measuring the
luminescence spectra.
10 QC mechanisms in Figure 11, the energy excited into levels
5
5
above G4 state would be relaxed to G4 state first, then part of
the energy was transferred to Yb3+ by CR3 and CR4,
completing the NIR QC process. In this process, the extra QE
for every phonon absorbed can be expressed by eqn (5)
15 assuming there does not exist nonradiative energy losses by
5
defects and impurities. The residual energy at G4 state would
be relaxed gradually to lower energy levels such as 5F3, and
undergo similar ET and emission processes as those under 450
nm excitation discussed above. This conclusion can be
20 verified by the drastic decrease of lifetimes at 5F3 and (5S2,
5F4) levels for Y5.34O5F8:10%Yb3+, 1%Ho3+ when excited at
360 nm, as shown in Figure S10 and Table 1. And this is why
the NIR emission spectrum exhibits a strong intensity around
1190 nm under 360 nm excitation.
55 Notes and references
Department of Materials Science, School of Physical Science and
Technology, Lanzhou University, Lanzhou 730000, P. R. China
Director of the Key Laboratory for Special Function Materials and
Structural Design of the Ministry of the Education, Lanzhou University,
60 Lanzhou 730000, P. R. China
*Corresponding author: wyh@lzu.edu.cn
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Figure 15 Emission spectra of Y6(1ꢀx)O5F8:xYb3+, 1%Ho3+ (0 ≤ x ≤ 15%),
inset shows emission spectrum of Y5.34O5F8:10%Yb3+, 1%Ho3+ in NIR
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